My wife and I were just having this discussion today.
The very fact of not having insurance scares people. And the pleasant feeling of having your expenses covered (or mostly covered) is something many people are willing to pay for. But keep separate the notions of "insurance" and "managed care."
Insurance is about managing risk. Dental cleanings, X-rays, and even fillings are not a risk. They are a certainty. If you pay for them through an insurance plan, you are guaranteed to pay more, (for the simple reason that you are paying more people to do their jobs for you).
Insurance is for things that you cannot afford to pay for ever. What dental risks are you envisioning? Imagine the worst possible dental problem that you could face. Orthodontics will cost several thousand dollars. Implants or facial surgery can be up to ten thousand. Now, you shouldn't like to pay so much, but if the remote possibility of a ten thousand dollar expense will ruin your life and your plans, then you probably aren't ready to FIRE anyway.
With medical insurance, this makes sense. You could get cancer or end up in the ICU for six months. On average you will pay more, but the average is at least not disastrous. I'm having trouble imagining an equivalent in dental though.
As far as routine expenses, you generally get two things: First, the insurance company does all the arguing about prices (and medical necessity) for you -- and they have more to bargain with (thousands of subscribers, instead of just one). And second, you don't have the hassle of actually arranging the payment.
On that last point, my own experience is that I very often end up calling insurance companies and providers anyway; waiting on hold; and then doing the negotiating between them that they should be handling on their own. So I don't get much value there. (My medical plan, on the other hand, is awesome: I just show up, get my medical care, and never ever see a bill or even know the price).
Why do I even have a dental plan? Because I'm not yet FIREd, and my employer paritally pays for it. After FIRE, I plan to have very high-deductible medical insurance, and probably no dental. I will probably continue to have collision insurance on my vehicles because I appreciate the convenience and I'm willing to pay statistically slightly more for it.